1. Field of the Invention
The present invention relates to crossed-field devices such as magnetrons, and more particularly, to an inductive insert for a magnetron that compensates for the natural increase in cavity inductance with temperature which causes the output frequency to decline with increasing temperature.
2. Description of Related Art
Magnetrons are a type of crossed-field device that are commonly used to generate high power microwave energy for assorted applications, such as radar. A magnetron typically comprises a cylindrically shaped cathode that extends axially along a central axis of an anode structure comprising a plurality of anode vanes that extend radially from an annular anode ring. A space defined between the cathode surface and the anode structure provides an interaction region, and an electric potential is applied between the cathode and the anode forming a radial electric field in the interaction region. An axial magnetic field is provided in the interaction region in a direction perpendicular to the electric field by pole-pieces that focus magnetic flux from magnets disposed externally of the interaction region. The cathode may be provided with an internal heater disposed below the surface of the cathode to heat the cathode surface to a temperature sufficient to cause thermionic emission of electrons therefrom. The emitted electrons are caused to orbit around the cathode in the interaction region due to the resultant forces derived from the crossed electric and magnetic fields, during which they interact with an electro-magnetic wave that is caused to move on the anode structure. The orbiting electrons give up energy to the electromagnetic wave, thus resulting in a high-power microwave electromagnetic wave circulating around the anode structure.
The anode structure comprises a plurality of resonant cavities. Each cavity is defined as the space bounded by the sides of adjacent vanes, the corresponding inner surface of the anode ring and the exterior of the interaction region. In the desired mode of operation, alternate anode vanes are at the same RF potential. Accordingly, the anode structure of magnetrons of the xe2x80x9cVane and Strapxe2x80x9d type further include straps that respectively couple alternating ones of the anode vanes to keep them at the same RF potential. Other types of magnetrons exist that do not use straps.
In order to put the high-power microwaves to use, an output circuit is provided to couple into the electric or magnetic (or both) fields that are supported on the anode structure and associated cavities. The output circuit can itself be coupled to a transmission system which takes the energy to the point of use. A typical transmission system includes waveguide. A typical output circuit includes a wire loop disposed in one of the cavities of the anode defined between adjacent anode vanes.
In the absence of any compensation, the natural increase in cavity inductance with temperature (in conjunction with several other less significant effects) causes the magnetron""s output frequency to decline with increasing temperature: the rate of decline is referred to as the temperature coefficient. This coefficient is always negative for an uncompensated magnetron and is principally derived from the thermal expansion coefficients of the materials used in the cavity walls (typically the anode ring and vanes).
In accordance with the teachings of the present invention, the rate of change of cavity inductance versus temperature (and by reference the temperature coefficient) can be selected at the design stage. Hence, this invention enables magnetrons to be designed with a wide range of rates of change of cavity inductance. Potentially, using this invention in any particular magnetron, the rate of change of cavity inductance can be made to both increase over that of the uncompensated magnetron or to decrease, even to the extent of becoming negative (i.e., where the cavity inductance actually reduces with increasing temperature).
The invention provides the capability of selecting the rate of change of inductance to be negative with increasing temperature which means that the temperature coefficient of frequency of the entire magnetron can be reduced below levels achieved by current design techniques to substantially zero. In other words by over-compensating the cavity inductance the change in cavity inductance and the other frequency-lowering effects can be entirely counteracted. One such other frequency-lowering effect would be a permanent magnet""s temporary loss of magnetic field strength as temperature increases because a lower magnetic field strength often causes the output frequency of the magnetron to drop.
The invention comprises inductive inserts within the cavities of a magnetron oscillator. The metals from which the cavity and inserts are made, and their volumes, can be selected to cause the natural increase of cavity inductance with increasing temperature to be reduced, nullified and even reversed without any manual manipulation. The selection of the materials depends on the value of the coefficient of thermal expansion.
These and other objects of the present invention are achieved by a microwave source including a magnetron that has an anode ring concentrically disposed around and spaced from a cathode. The anode ring includes a plurality of anode vanes with cavities being defined between adjacent ones of the plurality of anode vanes. The cavities have inductive inserts selected such that the rate of change of cavity inductance is compensated as temperature varies. The inductive inserts are themselves thermally and mechanically attached to the anode structure.
Still other objects and advantages of the invention will become readily apparent to those skilled in the art from the following detailed description, wherein the preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description thereof are to be regarded as illustrative in nature, and not as restrictive. In particular the present invention is not limited by the particular shape of the insert, nor by the number per cavity, the present invention relates to the thermal coefficients of expansion of materials chosen and the volume of each cavity and the total volume of all inductive inserts per cavity. The accompanying figures and description of the preferred embodiment show two inserts per cavity that are supported at one axial end in a xe2x80x9cvane and strapxe2x80x9d style magnetron with a loop-style output coupling element; the present invention is not limited to inserts of the shape shown; the present invention is not limited to two inserts per cavity; the present invention is not limited by the method of supporting the insert; the present invention is not limited to xe2x80x9cvane and strapxe2x80x9d style magnetrons; the present invention is not limited by the type of output circuit or output coupling element; the present invention is not limited by the transmission system.